[PatrickHenry]

Actually, the absolute, undispute, inarguable answer to this [HOW OLD IS THE EARTH?] is you don't know. And neither do I. There are a lot of guesses, some of which are better founded than others.

Is it a "guess" that the earth is at least as old as its oldest rocks?

The oldest rocks on Earth, found in western Greenland, have been dated by four independent radiometric dating methods at 3.7-3.8 billion years. Rocks 3.4-3.6 billion years in age have been found in southern Africa, western Australia, and the Great Lakes region of North America. These oldest rocks are metamorphic rocks but they originated as lava flows and sedimentary rocks. The debris from which the sedimentary rocks formed must have come from even older crustal rocks. The oldest dated minerals (4.0-4.2 billion years) are tiny zircon crystals found in sedimentary rocks in western Australia.

Source: The Age of the Earth.

[Tribune7]

But that assumes the radiometric dating methods are accurately calibrated.

For the record, I pretty much assume the Earth is 4 billion years old. On the other hand I've always assumed fossil fuel came from the decayed remains of creatures long dead.